LZW压缩算法

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<?php
/**
* @link http://code.google.com/p/php-lzw/
* @author Jakub Vrana, http://php.vrana.cz/
* @copyright 2009 Jakub Vrana
* @license http://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
*/

/** LZW compression
* @param string data to compress
* @return string binary data
*/
function lzw_compress($string) {
 // compression
 $dictionary = array_flip(range("\0", "\xFF"));
 $word = "";
 $codes = array();
 for ($i=0; $i <= strlen($string); $i++) {
  $x = $string[$i];
  if (strlen($x) && isset($dictionary[$word . $x])) {
   $word .= $x;
  } elseif ($i) {
   $codes[] = $dictionary[$word];
   $dictionary[$word . $x] = count($dictionary);
   $word = $x;
  }
 }
 
 // convert codes to binary string
 $dictionary_count = 256;
 $bits = 8; // ceil(log($dictionary_count, 2))
 $return = "";
 $rest = 0;
 $rest_length = 0;
 foreach ($codes as $code) {
  $rest = ($rest << $bits) + $code;
  $rest_length += $bits;
  $dictionary_count++;
  if ($dictionary_count > (1 << $bits)) {
   $bits++;
  }
  while ($rest_length > 7) {
   $rest_length -= 8;
   $return .= chr($rest >> $rest_length);
   $rest &= (1 << $rest_length) - 1;
  }
 }
 return $return . ($rest_length ? chr($rest << (8 - $rest_length)) : "");
}

/** LZW decompression
* @param string compressed binary data
* @return string original data
*/
function lzw_decompress($binary) {
 // convert binary string to codes
 $dictionary_count = 256;
 $bits = 8; // ceil(log($dictionary_count, 2))
 $codes = array();
 $rest = 0;
 $rest_length = 0;
 for ($i=0; $i < strlen($binary); $i++) {
  $rest = ($rest << 8) + ord($binary[$i]);
  $rest_length += 8;
  if ($rest_length >= $bits) {
   $rest_length -= $bits;
   $codes[] = $rest >> $rest_length;
   $rest &= (1 << $rest_length) - 1;
   $dictionary_count++;
   if ($dictionary_count > (1 << $bits)) {
    $bits++;
   }
  }
 }
 
 // decompression
 $dictionary = range("\0", "\xFF");
 $return = "";
 foreach ($codes as $i => $code) {
  $element = $dictionary[$code];
  if (!isset($element)) {
   $element = $word . $word[0];
  }
  $return .= $element;
  if ($i) {
   $dictionary[] = $word . $element[0];
  }
  $word = $element;
 }
 return $return;
}
$data = "";
$compressed = lzw_compress($data);
var_dump($data === lzw_decompress($compressed));
?>

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